U.S. Pat. Nos. 4,393,199 and 4,483,978, the teachings of which are incorporated herein by references, are directed to a method of cationic polymerization of cyclic ethers in which a polyhydric alcohol, e.g., a diol, is mixed with a cyclic ether monomer(s) and an acid catalyst. Polyethers formed from oxetane and tetrahydrofuran (THF) monomers are also described, for example, in U.S. Pat. Nos. 4,405,762 and 4,707,540, the teachings of which are incorporated herein by reference.
Polymers produced by cationic polymerization of cyclic ethers are useful for forming cross-linked elastomers. Such elastomers are useful, for example, for forming elastomeric binders for high-energy compositions, such as propellants, explosives, gasifiers or the like. In high-energy compositions, the cross-linked elastomer carries and spatially immobilizes large amounts of solid particulates, such as fuel particulates and oxidizer particulates. The high-energy composition may also contain a plasticizer for the elastomer.
A problem with known methods of cationic polymerization, such as taught by above-referenced U.S. Pat. Nos. 4,393,199 and 4,483,978, is that it is difficult to adequately control the reactions. Generally, cationic polymerization involves initiation using a preinitiator which is a polyhydric alcohol, e.g., a diol, such as butanediol, in conjunction with an acid catalyst, such as borontrifluoride (BF.sub.3) or an etherate of borontrifluoride. Ideally, polymerization proceeds from each hydroxyl group of the alcohol, e.g., from both ends of a diol. By this process, the polyhydric alcohol becomes incorporated within the polymer molecule. Unfortunately, using the above referenced cationic polymerization methods, polymerization frequently proceeds from less than all of the hydroxyl groups of a polyol molecule; for example, if a diol is used as an initiator, a significant proportion of the polymer molecules have the diol at one of the termini of the polymer chain due to polymerization proceeding from one hydroxyl group only. Inconsistent polymerization patterns result in high polydispersity, i.e., variation in the chain length of the polymer molecules. Often, not all of the alcohol initiator is incorporated, resulting in a molecular weight significantly different than that desired.
Both the lack of uniformity iu chain length (high polydispersity) and lack of full incorporation of the polyol into the polymer chain are considered disadvantageous with respect to forming cross-linked elastomers, e.g., through curing with polyfunctional isocyanates. Terminal hydroxyl groups on the initiator polyol residues are likely to have substantially different reactivity with the crosslinker than have the terminal hydroxyl groups of cyclic ether residues. A primary hydroxyl group at the end of a straight chain alcohol can be expected, for example, to react much more readily with isocyanate groups than do sterically hindered hydroxyl groups of residues of substituted oxetanes. Thus, terminal initiator derived molecules may give rise to unpredictable cross-linking reactions.
Also, many mechanical and elastomeric properties of cross-linked elastomers depend upon the length of the polymer molecules between the cross links. High polydispersity may give rise to unpredictable mechanical and elastomeric properties of cross-linked elastomers.
Cationic polymerization may also be used for forming a polymer having a hydroxyl group at one end only. However, heretofore, it has been difficult to obtain polymer having a single terminal hydroxyl group and uniform chain length by the above referenced cationic polymerization method. Monofunctional (hydroxyl) polymer has application with respect to forming ABA triblock and A.sub.n B star polymers by block linking techniques, such as that taught in U.S. Pat. No. 4,806,613 issued Feb. 21, 1989 to Wardle, the teachings of which are incorporated herein by reference.
There remains a need, therefore, for improved methods of cationic polymerization of cyclic ethers which provide for greater control of the polymerization reaction.